Determine the most recently evaluated variable and save it to .mat-file - matlab

Question

Is there a way to find the most recently evaluated variable in Matlab? who or whos don't seem to time stamp the variables in the workspace.
Here is a use case. I would like to have a generic function 'probe()' that can be called anywhere in a Matlab script/code. I would like to save the most recently evaluated variable to a .mat-file without having to pass any custom parameter relating to the variable being saved. Is this possible?
ans comes close to what I am trying to achieve, but it will not be available as my code has variables on the left hand side in the assignments.

Answer 1

If you are doing this in the Command Prompt, what you can do is use this post by gnovice to retrieve the entire command history since you opened MATLAB to a text array. Once you do that, you simply search the second last row for the variable before the equals sign.... assuming you did a lhs statement. You also want to take into account that you are echoing the variable in the Command Prompt without a left-hand side statement. We can easily find this through regexp.
You need to search at the second last row of the text array because gnovice's code to capture the history requires an additional line of code. This code gets captured in the history and that's not what you want. As such, you need to look at the second last row / entry.
Therefore, do this:
history = com.mathworks.mlservices.MLCommandHistoryServices.getSessionHistory; %// From gnovice's post
historyText = char(history);
lne = historyText(end-1,:); %// Get second last line
%// Do a regexp and retrieve the text with or without the equals sign
var = regexp(lne, '\w*', 'match');
%// Get first match which is the variable before any symbols or just the word
var = var{1};
Here's a quick example. This was my complete command history before trying the above code:
>> A = rand(10,10);
>> B = A + 2;
>> D = B * 3;
After I run the above code, I get for var:
var =
D
Similarly, if we just evaluate the answer without assigning anything to the left-hand side:
>> A = rand(10,10);
>> B = A + 3;
>> A
Running the above code, I get:
var =
A
To finally end it all, if you want to save this variable to disk, you can use an eval statement to facilitate this:
>> name_of_file = 'file.mat';
>> eval(['save ' name_of_file ' ' var]);
The above code will take a file name that you specify... so in this case it'll be test.mat, then invoke the save command with var as the variable from the Workspace you would like to save.

Answer 2

This is a basic sketch how you can do it, using the function dbstack:
function probe
%// get filename of caller file and line where probe was called
lineANDfile = dbstack;
file = lineANDfile(end).file;
linenum = lineANDfile(end).line;
%// read caller m-file
mLines = textread(file, '%s','delimiter', '\n');
%// get line previous of the line where probe was called
mLinesUntilProbeCall = mLines(1:linenum - 1);
%// last non empty line before probe call -> line of interest
mLine = mLines{ find( ~cellfun(#isempty,mLinesUntilProbeCall),1,'last') };
%// get string (variable name) left of =
varnameCell = textscan(mLine,'%s');
%// isolate varnames
getFouts = regexp(varnameCell{1}{1},'(?<=\[).+?(?=\])','match');
if isempty(getFouts)
varnames = varnameCell{1}(1);
else
varnames = strsplit( getFouts{1},',');
end
%// create struct with varnames as fields
for ii= 1:numel(varnames)
probeOut.(varnames{ii}) = evalin('base',varnames{ii});
end
save('probeOut.mat','probeOut');
end
which you can call in a script like:
y = 5;
xz = 42;
probe %// empty lines before call allowed!
it will create a .mat-file with a struct probeOutand the field xz.
After loading the .mat-file again:
>> probeOut.xz
ans =
42
It will also work, if you have multiple output arguments:
y = 5;
[xz,z] = deal(42,5);
probe
your .mat-file will then look like:
The case
y = 5;
[xz] = deal(42,5);
probe
is also covered.